Spleen cells of untreated mice lyse BALB.RL male 1 (RL.1) leukemia cells in vitro. We have adopted this as a standard system for immunogenetic studies of natural killer (NK) cells. For our purposes we define the NK population as the cell set with RL.1 lytic capacity and surface phenotype NK-1 ion. In preliminary work we have (1) established methods to enrich this NK cell set from heterogeneous populations; (2) adapted the Protein-A-SRBC rosette assay, which allows recovery and enrichment of both marker-positive and marker-negative viable cells, for use in the Kk-1 system; (3) established the Nk-1 phenotypes of 20 inbred strains; (4) determined that Kk-1 phenotypes are not related to levels of NK activity in different mouse strains, and (5) obtained evidence of a pro-Nk-1 cell set inducible for Nk -1 expression in vitro, permitting various uses of differentiative induction assays in the Nk -1 system. We propose (1) to define further the surface phenotype of this NK cell set in terms of expression of known immunogenetic surface markers; (2) to determine the differentiative origin of the NK cell set and its developmental relation to other cell sets; (3) to study the segregation of the Nk-1a (Nk-1.1) allele, map the Nk-1 locus, and to find alterative Nk -1 alleles and Nk-1 alloantigens; and (4) to determine whether C-type viral components are involved in NK activity in this model system. Thus we aim to determine the ontogeny of the Nk-1 ion cell set and to elucidate the specificities of its RL.1 lytic activity.